Liquid crystals have a variety of phase states, such as a smectic phase arranged in a lamellar manner, a nematic phase and a cholesteric phase with a helical structure, wherein the nematic liquid crystals can be further classified according to the relative orientation of molecules, and the smectic phase is the most common arrangement manner, in which the molecules are arranged in layers. Another common phase is the cholesteric phase, or referred to as a chiral nematic phase, in which the orientation of each layer of molecules is slightly twisted from the adjacent layer, thereby forming a helical, twisted structure in natural state. When a voltage is applied to such a liquid crystal, a corresponding angle will be twisted in the direction of the applied voltage. Therefore, it can be used to control the passage of light.
A chiral compound can be used as a dopant to induce or enhance the helical twisting of a liquid crystal composition used in a liquid crystal display. At a specific temperature, the helical twisting power (HTP) value of a chiral additive that causes helical twisting of a pure enantiomeric nematic liquid crystal composition by using the chiral additive has the following relationship with the resulting helical pitch, p, of the nematic liquid crystal composition and the concentration, c, of the chiral additive in the nematic liquid crystal composition:
  HTP  =      1    pc  wherein the unit of the helical pitch, p, is μm; the concentration, c, of the chiral additive can be expressed as a molar fraction, a weight fraction or a weight percent without any unit, and 0-1 is generally used for a weight fraction (e.g., 1 wt % is expressed as 0.01 in weight fraction), and the optical purity of the chiral additive (enantiomeric excess) is generally very close to pure. It can be seen from the above-mentioned formula that a short helical pitch can be achieved by means of a large HTP value or a large addition amount.
The existing disclosed chiral additives are, for example, as described in patents WO 2002006195, WO 9800428, WO 2007039104, WO 2007039105 and WO 2005023742; however, the chiral additives in the prior art generally have a low HIP value, a low solubility or a complicated synthesis process.
In some cases, the liquid crystal composition is required to have a strong helical twisting power, i.e., a smaller pitch, p. If the HIP value of the chiral additive is low, a higher amount of addition is required so as to adjust to a desired smaller pitch, p, and a high amount of addition may bring about more negative effects to the electrooptical properties, such as a dielectric anisotropy (Δε), an optical anisotropy (Δn), a clearing point (Cp), a viscosity (γl), a driving voltage and a response time, of the nematic liquid crystal composition.
A low solubility in the liquid crystal composition can lead to unwanted crystallization at low temperatures; at this point, two or more different chiral additives need to be added to avoid crystallization; in addition, the addition of two or more different chiral additives must achieve mutual compensation of the twisting temperature coefficients of different chiral additives.
Therefore, it is very necessary to provide a chiral additive having a high HTP value which is easy to synthesize and has a good solubility in a liquid crystal composition.